Recently, there has been a strong demand for a reduction in carbon dioxide emissions in order to have environmental protection. The automobile industry expects that the introduction of electric vehicles (EV) or hybrid electric vehicles (HEV) will lead to a reduction in carbon dioxide emissions. Thus, intensive efforts are being made to develop a motor driving secondary battery which holds the key to the practical application of those electric vehicles. As for the secondary battery, attention is drawn to a lithium ion secondary battery which can achieve high energy density and high output density.
Recently, the use of various electric vehicles has been promoted with the expectation of solving environmental/energy issues. A secondary battery is being developed intensively as a vehicle-mounted power source, such as a motor driving power source, which holds the key to the widespread use of these electric vehicles. However, in order to ensure widespread use, it is necessary to increase the performance and reduce the cost of batteries. In addition, with an electric vehicle, it is necessary to bring the single-charge driving distance closer to that of a gasoline engine vehicle. Thus, batteries with higher energy density are in demand. In order for batteries to have a high energy density, it is necessary to reduce as much as possible battery members that are not directly related to a battery reaction. As a battery which allows saving of current collecting tab of a battery single cell or bus bar for connection between single cells, has very high volume efficiency, and is suitable for mounting in vehicles, a bipolar type secondary battery has been suggested. In a bipolar type secondary battery (also referred to as bipolar secondary battery), a bipolar type electrode in which a positive electrode is formed on one surface of a current collector and a negative electrode is formed on the other surface of a current collector is used. Furthermore, it has a structure in which plural bipolar electrodes are layered such that the positive electrode and negative electrode can face each other while being mediated by a separator containing an electrolyte layer. Accordingly, the bipolar type secondary battery forms one battery cell (i.e., single battery) consisting of a current collector, a positive electrode and a negative electrode present between current collectors, and a separator (i.e., electrolyte layer). Furthermore, for the purpose of having even higher performance, use of a resin in which a conductive filler is dispersed in a current collector has been suggested.
For a lithium ion secondary battery with the aforementioned constitution, high energy density is important as a basic characteristic in order to have storage of energy that is required for running automobiles. As a method for increasing the energy density of a battery, a method in which ratio of a positive electrode material and a negative electrode material within a battery is increased is known. In Patent Literature 1, a means for increasing energy density of a battery by lowering the relative ratio of a current collector or a separator is disclosed.